Monotonic Array

Easy
481.

Monotonic Array

EasyArrayMetaAmazonCapital One

Monotonic Array is an easy coding problem on arrays, asked in Amazon, Meta and Capital One interviews.

An array is monotonic if it is entirely non-increasing or entirely non-decreasing.

Given an integer array nums, return true if it is monotonic and false otherwise.

Example 1

Input
nums = [1,2,2,3]
Output
true
Explanation
Non-decreasing throughout.

Example 2

Input
nums = [6,5,4,4]
Output
true
Explanation
Non-increasing throughout.

Example 3

Input
nums = [1,3,2]
Output
false

Constraints

  • 1 <= nums.length <= 100000
  • -100000 <= nums[i] <= 100000
Write your solution here. Running it against the tests needs a free account — what you type comes with you.Sign in to run
/**
 * @param {number[]} nums
 * @return {boolean}
 */
var isMonotonic = function(nums) {
    // Write your code here
    
};

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Monotonic Array — Solution in Python, Java & C++

Monotonic Array solution explained: step-by-step approach, complexity and code in Python, Java, C++ and JavaScript. An easy problem, asked at Amazon and Meta.

Monotonic Array is an easy coding problem on arrays, asked in Amazon, Meta and Capital One interviews.

  • Difficulty: Easy
  • Topics: Arrays
  • Asked at: Amazon, Meta, Capital One
  • Time limit: 2 s
  • Memory limit: 256 MB
  • Languages: JavaScript, TypeScript, Python, Java, C++, C, C#, Go, Kotlin, Swift, Rust, PHP and Ruby

Problem statement

An array is monotonic if it is entirely non-increasing or entirely non-decreasing.

Given an integer array nums, return true if it is monotonic and false otherwise.

Example 1

Input: nums = [1,2,2,3]
Output: true
Explanation: Non-decreasing throughout.

Example 2

Input: nums = [6,5,4,4]
Output: true
Explanation: Non-increasing throughout.

Example 3

Input: nums = [1,3,2]
Output: false

Constraints

  • 1 <= nums.length <= 100000
  • -100000 <= nums[i] <= 100000

Hints for Monotonic Array

  1. Hint 1

    You do not need two passes — track both possibilities at once.

  2. Hint 2

    Keep two booleans, increasing and decreasing, both starting true.

  3. Hint 3

    A strict drop kills increasing; a strict rise kills decreasing. Equal neighbours kill neither.

How to solve Monotonic Array

Monotonic means "non-decreasing OR non-increasing". Test both hypotheses in a single pass and see whether either survives.

Approach

  1. Start inc = true and dec = true.
  2. For each adjacent pair, if nums[i] < nums[i-1] set inc = false; if nums[i] > nums[i-1] set dec = false.
  3. Return inc || dec.

Why it works

inc stays true exactly when no strict descent exists, which is the definition of non-decreasing; dec mirrors it. Equal neighbours satisfy both definitions, so they must clear neither flag.

Complexity

  • Time — O(n)
  • Space — O(1)

Pitfalls

  • Using <= / >= in the clearing conditions makes a flat array like [2,2,2] report false.
  • Deciding the direction from the first differing pair alone needs care when the array starts with a run of equal values.

Reference solution

Python

from typing import List

def isMonotonic(nums: List[int]) -> bool:
    inc = True
    dec = True
    for i in range(1, len(nums)):
        if nums[i] < nums[i - 1]:
            inc = False
        if nums[i] > nums[i - 1]:
            dec = False
    return inc or dec

Java

public static boolean isMonotonic(int[] nums) {
    boolean inc = true, dec = true;
    for (int i = 1; i < nums.length; i++) {
        if (nums[i] < nums[i - 1]) inc = false;
        if (nums[i] > nums[i - 1]) dec = false;
    }
    return inc || dec;
}

C++

bool isMonotonic(vector<int>& nums) {
    bool inc = true, dec = true;
    for (int i = 1; i < (int) nums.size(); i++) {
        if (nums[i] < nums[i - 1]) inc = false;
        if (nums[i] > nums[i - 1]) dec = false;
    }
    return inc || dec;
}

JavaScript

var isMonotonic = function(nums) {
    let inc = true, dec = true;
    for (let i = 1; i < nums.length; i++) {
        if (nums[i] < nums[i - 1]) inc = false;
        if (nums[i] > nums[i - 1]) dec = false;
    }
    return inc || dec;
};

Also on the editorial tab: C, C#, Go, Kotlin, PHP, Ruby, Rust, Swift, TypeScript.

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